The invention relates to a circuit arrangement for driving an occupant protection system gas generator whose ignition process can be influenced by a magnetic field, the protection system being particularly an airbag.
Gas generators for airbags with an ignition process controllable by a magnetic field are known, for example, from WO 98/33684, where the magnetic field serves to influence the viscosity of a damper fluid; this in turn influences the movement of a piston moved by the pressure in the combustion chamber arising after an initial ignition, which piston also influences the supply of further liquid or gas-type fuel into the combustion chamber. The piston controls the size of the flow aperture or generates a pressure acting on the fuel, which controls the fuel quantity flowing into the combustion chamber and thus the fuel burn-off, that is, the ignition process and thus the inflation behavior of the airbag. Here the magnetic field is generated by a coil arranged around the damping cylinder and is to feature a strength in accordance with the required burn-off speed. Magneto-rheological damping fluids, whose viscosity can be changed accordingly by a magnetic field, are introduced, for example, by the U.S. Pat. No. 5,284,330. The damping increases with increasing current and thus increasing magnetic field. With regard to the setup and operating mode of such a gas generator, supplementary reference is expressly made to WO 98/33684.
Furthermore, in future it will also be necessary to take into consideration propellant gases whose burn-off behavior can be influenced directly by the strength of an electric field. All gas generators with a technical property that can be directly changed through the strength of a magnetic field will be understood as being gas generators whose ignition process can be influenced by a magnetic field. When coils are used as energy stores or for transformation purposes in the ignition current circuit, they feature a purely electric and exactly no such field action on the gas generator.
In principle, for the energy-efficient supply of smaller loads with a specifiable supply voltage or a supply current, for example from Tietze/Schenk: xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d (xe2x80x9cSemiconductor Circuit Technologyxe2x80x9d, 10th edition, Springer-Verlag 1993, pp. 563-564 switching controls are known where a circuit element and a storage inductance are connected in between input and outputxe2x80x94with current or voltage being determined on the load side, compared with a nominal value, and the circuit element controlled by pulse width modulation in relation to the comparison result. In the open switch position, a free-wheeling diode polarized opposite to the supply voltage acts as a free-wheeling branch for the storage inductance.
The invention introduces a suitable circuit arrangement for driving such a gas generator, with the said circuit arrangement providing for a magnetic field control which is as precise as possible. In addition, even in the event of a failure in the supply voltage infeed, triggering is still possible solely with a self-sufficient capacitor.
The operating mode of the circuit presented is based on the switch controller principlexe2x80x94but not its setup as instead of the storage inductance normally connected in between the input and output of switch controllers, the load coil generating the magnetic field is itself used as a component part of the switch controller. Besides saving the storage inductance which would otherwise be additionally required, the energy storage function of the load coil is additionally used to reduce the dissipation and thus the energy requirement; in particular, in cases of self-sufficiency this extends the self-sufficiency period or, for specified self-sufficiency period requirements, it also reduces significantly the required size of the self-sufficiency capacitor when compared with a simple application of switch controllers.
It needs to be taken into account here that the drive circuit and the gas generator are usually offered as separate components, even originating from different manufacturers, but that, nevertheless, the load coil in the gas generator becomes a functional part of the drive circuit.
Switch controllers are based on a pulse width modulated control of the circuit element, and an associated free-wheeling phase of the coil. It is conceivable that the pulse width ratio can be solely controlled in accordance with a specified nominal current without feedback. However, when embodied as a feedback control loop, the circuit will also detect the actual current through the coil, compare the same with a nominal value, and drive a circuit elementxe2x80x94connected in series to the coilxe2x80x94by pulse width modulation in relation to the result of the aforesaid comparison.
In addition, between circuit element and coil, a free-wheeling diode is connected in parallel to the coil. Via the switching frequency and the switching threshold, the strength of the magnetic field and the fluctuation range of the current can be specified exactly.
Preferably, a resistor that might be bridged over can be connected in series to the free-wheeling diode; if a reduction in the magnetic field of the coil is required, the magnetic field can be reduced even more quickly by means of this resistor. This is the case in particular if a significant acceleration in airbag inflation is required after the start of an accident sequence, and therefore the viscosity of the magneto-rheological damping fluid is to be reduced quickly. On the other hand, for charging and keeping the current constant, the bridging-over will keep losses during the free-wheeling phase to a minimum, which is particularly advantageous in self-sufficient operation due to the limited energy reserves for triggering the vehicle occupant protection device.
Alternatively, it is possible to feed back the energy stored in the coil into the self-sufficiency capacitor.